There is an increasing requirement in data and multimedia services for more and more local memory in hand-held or mobile devices. Presently, flash memories are the memory type typically used in mobile terminals such as mobile telecommunications devices. As data requirements increase, progress has been made to increase the capacity and decrease the cost of flash memories. However, there is still a need for a data storage medium having low cost, high storage density and high access speed.
Recently, probe storage systems have been developed. Probe storage utilises atomic force microscopy probes having tips which are heated so that when they make contact with a polymer surface of a recording medium, the heated tip softens the polymer surface and creates an indentation or “pit” in the polymer surface.
The probes are used for reading by exploiting the temperature dependent resistance of the probes. The probes are heated to a temperature lower than that required to melt the polymer. When the probe travels into a pit the heat transfer between the polymer and the probe is more efficient and the probe's temperature and hence resistance will decrease. The decrease in resistance is detected to detect the presence of the pit.
More recently developed probe storage devices, such as those shown in US 2003/0218960 or US 2004/0047275 use a storage medium and a probe array, wherein either the storage medium or the probe array is scanned in an x-y scanning directions. For example, the storage medium may be spring-mounted and can be pulled in the x and y direction by actuators on each edge. The storage medium moves below a two-dimensional array of fixed read/write probes. To access data, the medium is first pulled to a specified location. In addition, a feedback controlled z approaching scheme brings the probe array into contact with the storage medium. This contact is maintained and controlled while x-y scanning is performed for read/write. The array of probes, which may comprise thousands of probes, work simultaneously and each probe writes and reads information in its defined area. The probes thus scan their associated fields of the storage medium in parallel so that high data rates can be achieved. Already, such probe storage prototypes are demonstrating storage density as high as 3 Tb/inch2.
For performing accurate scanning of the recording medium, various types of operational data are required to be recorded on the recording medium. One type of operational data are servo marks patterns which are stored at certain locations on the probe storage medium to allow positioning and tracking of the probes during scanning. US 2003/0218960 discloses a method for scanning the storage media using a probe array, in which a servo mark pattern is located on the recording medium as shown in FIGS. 10 and 11. Four different types of patterns A, B, C and D are arranged on the probe storage medium as shown in FIGS. 10 and 11, either at intervals along one edge (FIG. 10) or equally spaced in a two dimensional grid (FIG. 11).
US 2004/0047275 discloses the use of a further type of operational data, being timing data used to adjust a clock used in the scanning. According to US 2004/0047275, for reasons of power conservation, periodic current or voltage pulses at short duration are applied to the probes, rather than DC current or voltage, for writing or reading. Accurate timing of the pulses is critical so that, for instance in reading, the pulses occur when the probes coincide with the centres of pits. Also, in the writing of data, the timing of writing pulses is important when writing to an “empty” or blank storage medium, but also when erasing a “dirty” or previously used medium. In an erasing operation, timing is important to time when pulses occur in order to melt out an indentation. Therefore, a reference clock pattern and a calibration field are used to correct the pulse frequency. The calibration field includes calibration data and the reference clock pattern is a pattern which is oversampled to adjust the clock frequency. FIG. 12 shows how the reference clock field and the calibration field are located in a corner of the storage medium in US 2004/0047275.
However, as shown in FIG. 13, forces applied to the storage media by the actuators cause distortion of the storage medium. This causes problems with the reading of the operational data and the accuracy of various calibrations carried out based on the reading of the operational data. For instance, if a target field for reading/writing data is located far away from the servo marks, then tracking will not be accurate based on the reading of the servo marks due to distortion of the storage media. Furthermore, if several fields of servo marks are used (A, B, C and D in FIGS. 10 and 11) which are spaced from each other, then distortion of the medium will affect the tracking or positioning accuracy.
Furthermore, the system may not detect or recognise servo marks or reference clock marks that are located at further distances from the media drivers because of location errors due to the distortion. With respect to the reference clock marks, there will be a higher probability of mismatch of position of reference clock and data marks where the data marks are further from the reference clock marks and further from the media drivers. Also, there will be a greater probability of non-detection of reference clock marks if these are located further from the servo marks.